cyclic voltammetric and spectroelectrochemical studies of ...from 75 to 300 mv.s-1 5are linear...

Int. J. Electrochem. Sci., 15 (2020) 10707 – 10721, doi: 10.20964/2020.11.66

International Journal of

ELECTROCHEMICAL SCIENCE

www.electrochemsci.org

Cyclic Voltammetric and Spectroelectrochemical Studies of

Tris(5-amino-1,10-phenantroline)Iron(II) Polymer Films

Kenneth L. Brown*, Rebecca Danforth, Evan Bleitz, Yechan Hwang and Dustin Rens

Hope College, Department of Chemistry, Holland, MI 494923

*E-mail: [email protected]

Received: 30 June 2020 / Accepted: 8 September 2020 / Published: 30 September 2020

Tris(5-amino-1,10-phenantroline)iron(II) polymer films were prepared on glassy carbon electrodes and

indium tin oxide (ITO) surfaces using cyclic voltammetry. The cyclic voltammograms collected during

electropolymerization are correlated with the electrochemical and spectroelectrochemical

characterization observations of the polymer films. On ITO surfaces, the films demonstrate reversible

electrochromism switching between colors of red and yellow due to potential changes either in the

cyclic voltammetry-scanning potential or pulsed-potential mode. Spectroelectrochemical

measurements of the polymer films were made using tetraethyl- and tetrabutyl-ammonium perchlorate

supporting electrolyte solutions. The absorbance at 506 nm is significantly diminished when

measurements are made in tetrabutylammonium perchlorate supporting electrolyte solutions,

indicating that spectroelectrochemical responses are limited by counterion diffusion within the

polymer films.

Keywords: Electropolymerization, cyclic voltammetry, spectroelectrochemistry, indium tin oxide,

polymer films

1. INTRODUCTION

Redox polymer films and organic conducting polymer films [1-3] have been important

components to function as selective reagents in compact, robust, and sensitive electrochemical sensors

and biosensors capable of detecting a wide variety of analytes. Cyclic voltammetry still remains a

versatile technique used to design polymer films for electrochromic devices for various types of panel

displays and electrochemical sensors and biosensors. In addition, these types of polymer films have

been used to develop neuroelectrodes and they can prevent corrosion and improve super capacitors [4-

6]. Although this method is effective for immobilizing compounds to an electrode surface, the polymer

film does not necessarily grow in regular layers, and hence, electrode surface coverage may require

several thousand monolayers to achieve complete surface coverage. This technique, under specified

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Int. J. Electrochem. Sci., Vol. 15, 2020

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experimental conditions (e.g., specific scan rates and potential windows), produces polymeric films

with consistent molecular architectures, mechanical stabilities, morphologies, and charge-transfer

properties [7-11]. Two significant advantages of using polymeric thin films as selective reagents in

chemical and electrochemical sensing are the ability to (1) improve control over the chemical and

physical nature of the reaction system and (2) provide a much higher effective "concentration density”

for reactions to occur. Spectroelectrochemistry offers the advantage of simultaneously probing the

electrochemical and optical properties of compounds affixed to transparent electrodes. Several groups

have exploited this hyphenated technique to develop flexible electrochromic materials and

spectroelectrochemical sensors [12, 13]. Meanwhile, electrochemical impedance spectroscopy (EIS) is

a versatile technique used to characterize and differentiate between mass transport and charge transfer

reactions occurring at electrode surfaces of redox species in solution and chemically modified

electrodes. Different types of diffusion, such as finite, semi-infinite, and infinite, which is based on the

time scale of the electrochemical experiment and thickness of the film, can impact the overall

electrochemical response of materials confined to surfaces when electrochemically stimulated [14, 15].

The diffusional fluxes of ions within the polymer film will give rise to a Warburg impedance that is

dependent upon the frequency of perturbation. We have used EIS to better understand the charge

transfer dynamics of tris(5-amino-1,10-phenanthroline)iron(II) [Fe(Phen-NH2)3] polymer films and to

determine the role of diffusion within these polymer films.

Tris(5-amino-1,10-phenanthroline)iron(II) electropolymerized films have been studied by

several research groups [16, 17] and have been used as redox mediators in glucose biosensors.

However, to date, no extensive study has been made on the spectroelectrochemical and electrochromic

properties of electropolymerized films derived from this compound. Furthermore, few applications of

using this compound in electrochemical systems have been noted. Therefore, in the development of

spectroelectrochemical sensors, we have revisited the electrochemical and spectroelectrochemical

properties of Fe(Phen-NH2)3 polymer films. In addition, atomic force microscopy and scanning

electron microscopy were used to study the morphological and optical properties of the

electropolymerized films on ITO surfaces.

2. EXPERIMENTAL

2.1 Reagents and Solutions.

Tris(5-amino-1,10-phenanthroline)iron(II) was synthesized according to procedures published

by Neils and researchers with no changes therein [17]. WebMO Basic computational software

interfaced with the Windows 10 operating system was used to build a computer model of [Fe(Phen-

NH2)3]. Acetonitrile, used as a solvent in all solutions, was purchased from Sigma Aldrich (St. Louis,

MO). Solutions of the iron complex were maintained at 1.00 x 10-3 M in 0.100 M tetraethylammonium

perchlorate (TEAP). Residual amounts of water in the electropolymerization solution compromises the

ability to successfully form the polymer film, as the residual water lowers the solubility of the iron

complex. As a result, prior to using TEAP or TBAP, which were obtained from GFS Chemicals


10709

(Powell, OH), the salts were dried under vacuum to remove the residual water and stored in a

desiccator until use. Glassy carbon electrode polishing and ITO surface cleaning are described

elsewhere [2].

2.2 Procedure for electropolymerization on glassy carbon electrodes and ITO surfaces and cyclic

voltammetric and EIS characterization of poly-Fe(Phen-NH2)3 films.

All electrochemical measurements were made using a CHInstruments (Austin, TX) or a

PalmSens (UK) potentiostat in a three-electrode cell configuration. The working electrode was either a

glassy carbon electrode (0.0706 cm2) obtained from CHInstruments or an ITO covered glass substrate.

The ITO surfaces used for spectroelectrochemical studies had dimensions of 7 mm x 50 mm x 0.6 mm

(width, height, and thickness) and were coated on one side of a glass substrate; a resistance of < 10

ohms was obtained with a system from Delta Technologies (Stillwater, MN). A platinum electrode

from CHInstruments served as the auxiliary electrode. The reference electrode obtained from

Innovative Instruments Inc. (Tampa, FL) was solid-state Ag/AgCl (3.4 M KCl) and leak-free.

Electropolymerizations were completed using a scan rate of 100 mV.s-1 in a potential window of 2.00

V to 0 V. The films on the glassy carbon electrodes and ITO surfaces were characterized at variable

scan rates from 10 mV.s-1 to 100 mV.s-1 in a potential window of 2.00 V to 0 V in the supporting

electrolyte solutions. Impedance measurements were made using glassy carbon electrodes at a DC

potential of 1.30 V and an AC amplitude potential of 10 mV vs Ag/AgCl reference electrode using a

frequency range of 0.001 Hz-50000 Hz.

2.3 Procedure for spectroelectrochemical and surface studies.

Spectroelectrochemical measurements were made with a Cary 5E UV/VIS/NIR

spectrophotometer using a three-electrode cell system in a 1.00-cm glass cuvette. A specially made

spectroelectrochemical cell top was designed to accommodate the three-electrode cell in the 1.00-cm

glass cuvette. The spectroelectrochemical measurements of the polymer films on ITO surfaces made in

the cyclic voltammetry mode at 25 mV.s-1 (2.00 V to 0 V) and pulse-based mode were based on twenty

cycles of electropolymerization on the ITO surface. In the pulse-based mode, applied potentials were

held for 1 minute at +1.50 V, +1.35 V, +1.30 V, +1.25 V and +1.20 V before a spectrum was obtained

for the polymer film. All the measurements were made in 0.100 M TEAP or 0.100 M TBAP

supporting electrolyte solutions. Scanning electron microscopy measurements were made using an

EM-30 series instrument (Element Precision Imaging). A Dimension Icon atomic force microscope

(Bruker, Santa Barbara, CA) was used to obtain the film thickness of the polymer film on the ITO

surface.


10710

3. RESULTS AND DISCUSSION

3.1 Electropolymerization of tris(5-amino-1,10-phenanthroline)iron(II) on glassy carbon electrodes.

Figure 1 shows the structure of Fe(Phen-NH2)3 drawn using WebMO computational software

and cyclic voltammograms corresponding to twenty cycles of the electropolymerization of Fe(Phen-

NH2)3 on a glassy carbon electrode [18]. Regardless of the working electrode used (i.e., glassy carbon

or ITO) the mechanism of forming these films commences with the irreversible oxidation of the amine

substituent at 1.18 V on the glassy carbon electrode to produce radical cations that combine at the

electrode surface. The coupling reaction is followed by the loss of protons, providing a neutral species.

Chain propagation occurs via the addition of more monomer units through radical cation coupling,

which causes the insoluble polymeric material to become deposited onto the electrode surface. Other

compounds, such as metal(II)-tetraaminophthalocyanines, thiophene, 3,4-ethylenedioxythiophene

(EDOT), and aniline, are known to proceed with similar mechanisms to form their corresponding

polymer films [10, 19]. However, the peak due to amine oxidation eventually becomes overlapped by

the growth of the peak corresponding to Fe2+→Fe3+ + e-. The main redox reaction, Fe3+ ⇌ Fe2+ + e-,

produces the anodic peak (Epa) and cathodic peak (Epc) with Eo' of 1.25 V (glassy carbon electrode),

where Eo', the formal reduction potential, is the average of the Epc and Epa peak potentials. The increase

in the peak currents during electropolymerization results from the accumulated monolayers deposited

on the electrode surface and increased surface area related to the changing surface roughness.

Figure 1. The structure of Fe(Phen-NH2)3 and twenty cycles of the electropolymerization of

1.00 x 10-3 M Fe(Phen-NH2)3 in 0.100 M TEAP at 100 mV.s-1 using a potential of 0.00 V to

2.00 V vs Ag/AgCl are shown.

Figure 2 shows an overlay of the cyclic voltammograms corresponding to cycles one and

twenty on a glassy carbon electrode. The peak separation of the Fe2+/3+ redox couple during

electropolymerization varies from 0.090 V to 0.110 V on glassy carbon electrodes. At the onset of

electropolymerization, the ratio of the peak currents (ipa/ipc) is 1.53, but afterwards, this ratio is 0.990-

1.09.


10711

Figure 2. The differences between cycle one and cycle twenty are shown for the electropolymerization

of Fe(Phen-NH2)3 on a glassy carbon working electrode. Solution conditions are the same as in

Figure 1.

Figure 3a shows an overlay of the cyclic voltammograms from 75 mV.s-1 to 275 mV.s-1. The

diagnostic plots shown in Figure 3b of the peak currents with respect to the square root of the scan rate

from 75 to 300 mV.s-1 are linear according to the Randles-Sevcik equation, ip =2.69x105n3/2ACD1/2v1/2,

indicating diffusion-controlled mass transport conditions during the electropolymerization process

[20]. The symbols have their common electrochemical representation.

Figure 3a. The overlay of the cyclic voltammograms of the electropolymerization of 1.00 x 10-3 M

Fe(Phen-NH2)3 in 0.100 M TEAP conducted at scan rates from 75 to 275 mV.s-1 using a

potential of 0.00 V to 2.00 V vs Ag/AgCl.


10712

Figure 3b. A plot of the peak currents with respect to the square root of the scan rates from 75 to 275

mV.s-1, showing the diffusion-controlled conditions for electropolymerization.

3.2 Electropolymerization of tris(5-amino-1,10-phenanthroline)iron(II) on indium tin oxide surfaces.

The electropolymerization of Fe(Phen-NH2)3 on ITO surfaces shown in Figure 4 parallels very

closely with the electropolymerization of Fe(Phen-NH2)3 on GC electrodes. The initial oxidation of the

-amino group occurs at 1.08 V. However, the peak separations of the Fe2+/3+ redox couple for each

cycle of electropolymerization are much greater than those obtained with the GC electrodes. During

the first cycle, the peak separation is 276 mV, whereas on the GC electrode, the peak separation is 90

mV. As the electropolymerization process continues, ∆Ep/cycle on ITO surfaces varies linearly from

276 mV to 575 mV. Due to the ohmic resistance, the electrochemistry of Fe(Phen-NH2)3 on ITO

surfaces shows a more quasi-reversible behavior than that on the glassy carbon electrodes, although the

films are formed on the ITO surface with the same ease as the films formed on the glassy carbon

electrodes; the peak separation values are relatively constant on GC electrodes.

3.3 Cyclic voltammetric characterization of poly-Fe(Phen-NH2)3 films.

Examination of the GC electrodes after electropolymerization revealed a red film that increased

in color intensity as the surface coverage increased. When the electrode was rinsed with fresh solvent,

placed in the supporting electrolyte solution, 0.100 M TEAP in CH3CN, and characterized (Figure 5),

the cathodic and anodic peaks persisted upon scanning from 2.00 V to 0 V at 25 mV.s-1. The Eo 'value

of Fe3+ ⇌ Fe2+ + e- is 1.37 V, which is similar to the Eo' value of the Fe3+/2+ redox couple obtained

during the electropolymerization cyclic voltammetry. A surface coverage of 5.10 x 10-7 mol/cm2 was

determined using the area under the anodic peak. The cathodic and anodic peaks in Figure 5 are the

result of several processes: 1) charge transfer at the electrode|polymer interface, 2) electron hopping

between nearby redox centers throughout the polymer film, 3) egress or ingress of ions from the film


10713

Figure 4. Cyclic voltammogram shows twenty cycles of the electropolymerization of 1.00 x 10-3 M

Fe(Phen-NH2)3 in 0.100 M TEAP at 100 mV.s-1 using a potential of 0.00 V to 2.00 V vs

Ag/AgCl on an ITO surface.

via the electrolyte supporting electrolyte solutions to maintain charge electroneutrality, and 4) possible

changes in the polymer network to accommodate charge transfer reactions and ionic movement within

the polymer structure [21]. During the oxidation and reduction reactions, solvated counterions enter or

leave the polymeric network to maintain charge electroneutrality and may cause the film to swell, thus

changing the mechanical integrity of the film [22, 23]. Diagnostic plots of the peak current relative to

the scan rate produced linear plots, indicating thin film electrochemistry, wherein finite diffusion

dominates the electron transfer process within the films [24, 25].

Figure 5. The characterization of the poly-Fe(Phen-NH2)3 film on a glassy carbon electrode in

0.100 M TEAP at 25 mV.s-1 using a potential of 0.00 V to 2.00 V vs Ag/AgCl.


10714

Scan rates less than 25 mV.s-1 produce peak separations of practically 0 V, indicating Nernstian

equilibrium with the applied potential; that is, increasing the scan rates causes the peak separation to

increase, as semi-infinite diffusion conditions dominate the system, and the cyclic voltammetric scan

rate is high relative to the rate of charge transfer. In addition, at scan rates less than 25 mV.s-1,

ipa/ipc=1.00, but the peak width at half-height is greater than the predicted value of 90.4 mV/n,

indicating the presence of nonequivalent redox centers and/or attractive or repulsive forces present

within the film [25]. The polymer film is essentially a three-dimensional network of redox centers

surrounded by solvent pockets devoid of counterions in some areas of the film but present in other

locations. Hence, the local activity of the film changes as distance from the electrode surface increases.

This also points to the changing activity of [Fe2+]/[Fe3+] as time varies during the cyclic voltammetry

scan [26].

Figure 6 shows the characterization of the films on ITO surfaces corresponding to 10 mV.s-1

and 25 mV.s-1 scan rates. Using these scan rates, the peak currents increased by a factor of 2.66. A

complete plot of the peak currents with respect to the scan rate produced a linear trend, indicating the

thin film electrochemistry of the polymer film on the ITO surface. In addition, the Eo' values obtained

for Fe3+/2+ of the films on the ITO surfaces and glassy carbon electrodes are very similar, with ITO

films having an Eo' value of 1.30 V. The larger current response is a result of the larger surface area of

the ITO surface relative to the glassy carbon electrode.

Figure 6. The characterization of a poly-Fe(Phen-NH2)3 film on an ITO surface in 0.100 M TEAP at

25 mV.s-1 using a potential of 0.00 V to 2.00 V vs Ag/AgCl.

3.4 Spectroelectrochemical and SEM characterization of poly-Fe(Phen-NH2)3 films.

Spectroelectrochemical measurements in the cyclic voltammetric mode and constant potential

mode demonstrate the electrochromic properties of the polymer films on ITO surfaces, switching


10715

between the colors of red and yellow. The overlay spectra obtained after applying potentials of 1.50 V,

1.35 V, 1.30 V, 1.25 V, and 1.20 V are shown in Figure 7. At a potential of 1.50 V, the predominant

redox state is Fe3+ in the polymer film, and the film is yellow. However, as the potential becomes more

negative, an absorption band begins to appear at ca. 506 nm and reaches a maximum absorbance of

0.500 at 1.20 V. At this voltage, the film switches to red, wherein the predominant redox state is Fe2+.

An isosbestic point appears at 412 nm, indicating the presence of two principle species, Fe2+/Fe3+, in

Nernstian equilibrium and with equal molar absorptivity values. In addition, the cyclic voltammetric

characterization shown in Figure 5 also indicates facile electron transfer and Nernstian equilibrium of

the Fe3+/Fe2+ oxidation states with the applied potential at low scan rates. Using the absorbance values

(A2-Aox)/(Ared-A2) to plot the applied potential with respect to log [O]/[R] yields an estimation of Eo' in

0.100 M TEAP, where A2 represents an absorbance measurement, and Aox and Ared are the absorbance

measurements of the oxidized and reduced states, respectively [27]. The equation of the line provided

an intercept of 1.31 V, which is Eo'; this coincides with the Eo' value determined by the cyclic

voltammetric measurements shown in Figures 5 and 6.

Figure 7. The overlay spectra obtained from the spectroelectrochemical measurements made in the

pulsed-potential mode of the poly-Fe(Phen-NH2)3 film in 0.10 M TEAP in acetonitrile. The

polymer film is based on twenty cycles of electropolymerization. Applied potentials vs

Ag/AgCl from the bottom to the top are 1.50 V, 1.35 V, 1.30 V, 1.25 V, and 1.20 V. A

platinum wire served as the auxiliary electrode.

Electrochromic materials can be characterized at a specific wavelength in terms of optical or

chromic contrast (∆T), optical density (∆ODλ), and electrochromic or coloration efficiency (CE).

These parameters [13, 28] are defined by equations (1)-(3). The optical contrast parameter shown in

equation (1) is the difference in the transmittance of the bleached (Tb) state and colored (Tc) state of

the polymer film:

∆T=Tb-Tc (1)

The optical density factor at a specific wavelength is given by equation (2):


10716

bλ

c

TΔOD =log

T (2)

The electrochromic or coloration efficiency shown in equation (3) is described by two terms,

the optical density and the charge inserted (Qin) per unit area (A) to the initiate the switch in the redox

state:

λΔOD

in

CEQ

= (3)

where the value of Qin was determined by the characterization cyclic voltammogram:

dtin

iQ

A= (4)

Figure 8a corresponds to the spectroelectrochemical measurements performed at 506 nm using

the cyclic voltammetric mode and 0.100 M TEAP as the supporting electrolyte system, wherein the

width of each cycle is dependent upon the scan rate. When the film is in the oxidized state (Fe2+ →

Fe3+ + e-), the absorbance obtains a minimum value. However, as the potential is made more negative

to realize the reduction process (Fe3+ +e-→ Fe2+), the absorbance reaches a maximum. This oscillation

continues with little change in the maximum absorbance. Using ∆ODλ and Qin values of 0.575 and

0.487 mC/cm2, respectively, the electrochromic efficiency is 1180 cm2/C. These values are based on

using 100%∆T, although some groups have used 95%∆T. Other research groups have cited

electrochromic efficiency values of conjugated polymer systems within the range 500-3000 cm2/C

[29].

Figure 8a. The spectroelectrochemical characterization of a poly-Fe(Phen-NH2)3 film on an ITO

surface measured at 506 nm and 25 mV.s-1 vs Ag/AgCl reference electrode in 0.100 M TEAP.

0

0.1

0.2

0.3

0.4

0.5

0.6

0 200 400 600 800 1000 1200 1400

Ab

sorb

an

ce

Time (sec)


10717

Figure 8b. The spectroelectrochemical characterization of a poly-Fe(Phen-NH2)3 film on an ITO

surface measured at 506 nm and 25 mV.s-1 vs Ag/AgCl reference electrode in 0.100 M TBAP.

The impact of the supporting electrolyte system on the electrochromic properties of the

polymer film was evaluated using 0.100 M TBAP. Upon the reduction of the polymer film, the ClO4-

counterion is released from the film, or, concomitantly, TEA+/TBA+ ingresses into the film. The

significantly reduced absorbance, and, hence, the lower electrochromic efficiency shown in Figure 8b

for the TBAP-based system are attributed to the difficulty of TBA+ to provide charge electroneutrality

within the polymer film upon redox switching. The counter ion TBA+ is larger than TEA+ and hence,

the mobility of TBA+ within the polymer film is lower than TEA+. Therefore, the diffusion of ions in

the polymer film is the limiting factor for the overall electrochromic response of the polymer film. The

switching time for the polymer film is dependent upon the ratio, L2/D, where D is the ionic diffusivity,

and L is the film thickness [28]. Hindrances to ionic diffusion within the polymeric network limit the

switching time and overall performance of the polymer film as an electrochromic material. The

switching times for the reduction and oxidation processes are 3.5 seconds and 7.2 seconds,

respectively, based on twenty cycles of electropolymerization.

Figure 9 shows an SEM image obtained with 2500X magnification of a polymer film on an

ITO surface undergoing twenty cycles of electropolymerization. The different levels of gray features in

the image represent the topographical, structural, and porous characteristics of the polymer film. In

addition, the porous nature of the polymer film is indicative of a material that does not form in regular

layers during the electropolymerization process, as any solvent pockets present within the film are

randomly distributed. No doubt, the redox centers throughout the monolayers are nonequivalent or

attractive, or repulsive forces are present [25, 26]; this is confirmed by the broad characterization

cyclic voltammogram of Figures 5 and 6. The film thickness was obtained from AFM measurements

by averaging the height differences between the ITO surface and the film after scratching the polymer

film. The width of the scratch through the film was approximately 20 µm, and the glass substrate was

undamaged. The film thickness corresponding to 20 cycles of electropolymerization is 462 ± 45 nm

(n=4).

0

0.05

0.1

0.15

0.2

0.25

0 1 2 3 4 5 6

Ab

sorb

an

ce

Time (min)


10718

Figure 9. Scanning electron microscopy image obtained at 2500X magnification of a poly-Fe(Phen-

NH2)3 film on an ITO surface undergoing twenty cycles of electropolymerization.

3.5 General impedance characterization of poly-Fe(Phen-NH2)3 films.

Evidence of mixed diffusion and the charge transfer impedances is shown in Figure 10,

exhibiting a Nyquist plot of a poly-Fe(Phen-NH2)3 film [13]. The Nyquist plot shows a charge transfer

resistance, given by the semicircle at high frequencies of 50000 Hz. Furthermore, at the terminal end

of the charge transfer resistance, 181 Hz, a Warburg impedance develops, which results from the

diffusion of ions within the polymer film [29]. As the film becomes thicker, the diffusion process

becomes anomalous, and the Warburg impedance component deviates from the 45 phase angle [14].

It has been shown that thicker films have a greater surface roughness than thinner films, which leads to

the greater trapping of ions. Upon redox switching, there is the possibility that ions become trapped

within the shallow or deep regions of the polymer film, which imposes limitations on the optical

response of the films [30]. The polymer film can be considered as a structure with dense compact

layers closer to the electrode surface and more porous layers near the polymer film|electrolyte

interface. Thicker films provide a more porous structure containing solvent pockets with or devoid of

ions randomly distributed throughout the polymeric network. More diffusion occurs within the thicker

porous structure as the ions have more residence time along the diffusional path segments, giving rise

to anomalous diffusion and a Warburg impedance [31]. Characteristically, thinner films also exhibit a

Warburg impedance, but with a normal diffusion path and a smaller resistance for charge transfer. No

doubt as the electrolyte interacts with the polymer film during redox switching, film swelling occurs,

which causes the film structure, the diffusional process, the pathway of ions, and the Warburg

impedance to change.


10719

Figure 10. The electrochemical impedance spectroscopy characterization showing the Nyquist plot of

the poly-Fe(Phen-NH2)3 film on a glassy carbon electrode in 0.100 M TEAP. The parameters

include the following: a 1.30 V DC potential and a 10 mV AC potential amplitude vs Ag/AgCl;

0.001 Hz to 50000 Hz.

Electropolymerized films of Fe(Phen-NH2)3 have never been characterized using

spectroelectrochemical, electrochemical impedance spectroscopy, and microscopy methods to

understand the charge transfer processes occurring within these films. The results of characterizing the

Fe(Phen-NH2)3 films on glassy carbon and ITO surfaces with these methods have been correlated to

each other. Due to the differences in the spectroelectrochemical response of the films in the TEAP and

TBAP supporting electrolyte solutions, the films hold promise as electroactive immobilized reagents in

spectroelectrochemical sensors and electrochromic devices. In comparison to other polymer film

systems, electropolymerized films of metal(II)-tetraaminophthalocyanine also exhibit similar

electrochromic responses when exposed to TEAP and TBAP supporting electrolyte solutions [32].

Furthermore, electrochromic devices that are based on the copolymerization of EDOT and

phenylthiophene compounds on ITO surfaces have electrochromic efficiency values ranging from 15.6

- 152 cm2/C [33]. With EDOT substituents, multiple redox processes can occur, giving rise to various

colored states. The cyclic voltammetric response of poly-Fe(Phen-NH2)3 is very similar to that of other

ligand systems with iron as the redox center [34]. Iron and ruthenium porphyrin complexes consisting

of -amino and -hydroxy substituents have been electropolymerized on glassy carbon and platinum

electrodes to prepare multilayered polymer films, which exhibited facile electron hopping between

redox centers when characterized using cyclic voltammetry in various supporting electrolyte solutions.

In addition, in most of these systems, the peak width at half-height is greater than the predicted 90.4

mV/n value; however, the film thicknesses are not reported [34].

4. CONCLUSIONS

Electropolymerized films of Fe(Phen-NH2)3 are formed with ease on both glassy carbon

electrodes and ITO surfaces and exhibit good electrochemical reversibility at the glassy carbon

0

200

400

600

800

1000

0 200 400 600 800 1000

-Z''

(oh

ms)

Z' (ohms)


10720

electrode. The films are stable upon prolonged electrochemical stimulation in supporting electrolyte

solutions and are not removed from glassy carbon or ITO surfaces. The films demonstrate reversible

electrochromism that is very comparable to other systems, both organic conducting polymer films and

redox films, in terms of efficiency. However, the electrochromic properties, such as a switching time

of several seconds, are dependent upon the ion mobility within the polymer films. Although this

switching time value is beyond the millisecond time scale, our values are based upon a film thickness

greater than 400 nm. Fine tuning the electropolymerization process through a simplex optimization

algorithm to better control the film thickness, film charge transfer resistance, and electrochromic

response is currently under study.

ACKNOWLEDGMENTS

Research supported by the National Science Foundation-Research Experience for Undergraduates

(NSF Grant Award #1263097) and the Hope College Chemistry Department Endowed Funds.

References

1. E. M. Baum, H. Li, T. F. Guarr and J. D. Robertson, Nucl. Instrum. Methods Phys. Res. B Beam Interact. Mater. At., 56-57 (1991) 761.

2. K. L. Brown and H. A. Mottola, Langmuir, 14 (1998) 3411. 3. S. W. Feldberg, J. Am. Chem. Soc., 106 (1984) 4671. 4. A. S. Widge, M. Jeffries-El, X. Cui, C. F. Lagenaur and Y. Matsuoka, Biosens. Bioelectron., 22

(2007) 1723.

5. S. L. Zelinka, L. Ortiz-Candelaria, D. S. Stone and D. R. Rammer, Forest Prod. J., 59 (2009) 77. 6. S. Mondal, T. Yoshida, S. Maji, K. Ariga and M. Higuchi, ACS Appl. Mater. Interfaces, 12 (2020)

16342.

7. P. Denisevich, K. W. Willman and R. W. Murray, J. Am. Chem. Soc., 103 (1981) 4727. 8. M. Yamada, N. Ohnishi, M. Watanabe and Y. Hino, Chem. Commun., 46 (2009) 7203. 9. B. Krische and M. Zagorska, Synth. Met., 33 (1989) 257. 10. J. M. Calvert, R. H. Schmehl, B. P. Sullivan, J. S. Facci, T. J. Meyer and R. W. Murray, Inorg.

Chem., 22 (1983) 2151.

11. K. Maksymiuk and K. Doblhofer, Electrochim. Acta, 39 (1994) 217. 12. S. A. Sapp, G. A. Sotzing and J. R. Reynolds, Chem. Mater., 10 (1998) 2101. 13. T. Xu, E. C. Walter, A. Agrawal, C. Bohn, J. Velmurugan, W. Zhu, H. J. Lezec and A. A. Talin, Nat.

Commun., 7 (2016) 10479.

14. G. Láng and G. Inzelt, Electrochim. Acta, 36 (1991) 847. 15. P. N. Bartlett and K. F. E. Pratt, J. Electroanal. Chem., 397 (1995) 61. 16. M. V. Martinez, R. Coneo Rodriguez, A. Baena Moncada, C. R. Rivarola, M. M. Bruno, M. C.

Miras and C. A. Barbero, J. Solid State Electrochem., 20 (2016) 2951.

17. W. M. F. Nyasulu and H. A. Mottola, J. Electroanal. Chem. Interfacial Electrochem., 239 (1988) 175.

18. J. R. Schmidt and W. F. Polik, WebMO Enterprise, Version 20.0, WebMO LLC, (2020) Holland, MI.

19. G. A. Sotzing, J. R. Reynolds and P. J. Steel, Chem. Mater., 8 (1996) 882. 20. P. T. Kissenger and W. R. Heineman, Laboratory Techniques in Electroanalytical Chemistry,

Marcel Dekker, (1996) New York, NY.

21. A. Akhoury, L. Bromberg and T. A. Hatton, J. Phys. Chem. B, 117 (2013) 333.


10721

22. W. Ogieglo, H. Wormeester, K.-J. Eichhorn, M. Wessling and N. E. Benes, Prog. Polym. Sci., 42 (2015) 42.

23. J. Troyano, A. Carné‐Sánchez, J. Pérez‐Carvajal, L. León‐Reina, I. Imaz, A. Cabeza and D. Maspoch, Angew. Chem., 130 (2018) 15646.

24. M. R. Nateghi, M. H. Mosslemin and H. Hadjimohammadi, React. Funct. Polym., 64 (2005) 103. 25. P. Daum, J. R. Lenhard, D. Rolison and R. W. Murray, J. Am. Chem. Soc., 102 (1980) 4649. 26. P. Daum and R. W. Murray, J. Phys. Chem., 85 (1981) 389. 27. E. L. D’Antonio, E. F. Bowden and S. Franzen, J. Electroanal. Chem., 668 (2012) 37. 28. S. Hassab, D. E. Shen, A. M. Ӧsterholm, M. Da Rocha, G. Song, Y. Alesanco, A. Viñuales, A.

Rougier, J. R. Reynolds and J. Padilla, Sol. Energy Mater. Sol. Cells, 185 (2018) 54.

29. M. Fabretto, T. Vaithianathan, C. Hall, J. Mazurkiewicz, P. C. Innis, G. G. Wallace and P. Murphy, Electrochim. Acta, 53 (2008) 2250.

30. V. F. Lvovich, Impedance Spectroscopy Applications to Electrochemical and Dielectric Phenomena, Wiley, (2012) New Jersey, NJ.

31. N. Maouche and B. Nessark, Int. J. Electrochem., 2011 (2011) Article ID 670513. 32. H. Li and T. F. Guarr, J. Electroanal. Chem. Interfacial Electrochem., 297 (1991) 169. 33. G. R. D. B. S. Lacerda, C. R. Calado and H. D. R. Calado, J. Solid State Electrochem., 23 (2019)

823.

34. A. Bettelheim, B. A. White, S. A. Raybuck and R. W. Murray, Inorg. Chem., 26 (1987) 1009.

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cyclic voltammetric and spectroelectrochemical studies of ...from 75 to 300 mv.s-1 5are linear...

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